This invention relates to a projection apparatus for projecting through an optical system a pattern formed on an object onto another object, and, more particularly, to a projection apparatus for use in the manufacture of semiconductor devices such as integrated circuits (ICs), large scaled integrated circuit (LSIs) and very large scaled integrated circuits (VLSIs).
In the field of semiconductor devices, miniaturization of the circuit pattern has been promoted in order to ensure high density integration of the devices. This promotion of miniaturization has forced development of improved exposure apparatuses for exposing a wafer to the circuit pattern formed on a mask or reticle to reproduce or print the circuit pattern on the wafer. Along with this trend, projection type exposure apparatuses are becoming dominant as compared with traditional contact type or proximity type exposure apparatuses. The projection type exposure apparatuses include projection optical systems such as mirror projection systems or lens projection systems.
The lens projection type exposure apparatuses generally include automatic focusing mechanisms for automatically locating the wafer surface at the focus position of the lens system. Most of these automatic focusing mechanisms are generally arranged such that a reference plane is set at a position spaced from the end surface of the lens system through a predetermined distance, the reference plane corresponding to the focus position of the lens system, and the wafer is moved so that the wafer surface is coincident with the reference plane, whereby the wafer surface is positioned at a constant distance from the end surface of the lens system. With such arrangement, the wafer surface can be coincident with the focus position highly accurately as long as the focus position of the lens system is fixed. If, however, the focus position of the lens system displaces for any reason, it would not be possible to make the wafer surface accurately coincident with the focus position.
This will now be described in more detail. In general, the critical resolving power L of the projection type exposure apparatus can be given by: EQU L=1.6.lambda.Fe . . . (1)
where .lambda. is the wavelength of the exposure beam and Fe is the F-number of the projection optical system. In order to improve the resolving power L, it is required to use a shorter wavelength .lambda. and/or to make the F-number Fe smaller. On the other hand, the depth of focus D of the optical system is given by: EQU D=.+-..lambda.Fe.sup.2 (.lambda./8 standard) . . . (2)
Therefore, an increase in the resolving power L, i.e. decrease in the wavelength .lambda. and/or F-number, makes the depth of focus smaller. Usually the projection exposure apparatus uses a wavelength of g-line (.lambda.=436 nm) and an F-number of approx. 1.43. The depth of focus D in such case is only .+-.0.9 micron. This results in that, if, in the lens projection type exposure apparatus employing the above-described automatic focusing mechanism, the focus position of the lens system is deviated for any reason, the circuit pattern could not be accurately projected on the wafer surface.
Factors for the deviation of the focus position of the projection optical system may be (1) changes in the temperature of the air between the reticle and wafer and changes in the temperature of the glass materials in the projection optical system, (2) changes in the pressure of ambient air between the reticle and wafer and (3) changes in the humidity of the air between the reticle and wafer.
As regards the point (1), variable elements in the components of the projection optical system may be the radius of curvature of the lens surface, the distance between the lens surfaces, and the relative index of refraction defined by the air and glass material. Changes in these elements would result in deviation of the focus position of the projection optical system. Coefficiently, the temperature change among the aforementioned three factors will cause greatest amount of focus position change. Traditionally, air-conditioning means has been employed to control the temperature within the exposure apparatus and to control the environment conditions of the apparatus to thereby suppress the amount of focus position change.
On the other hand, as regards the points (2) and (3) with respect to the changes in the pressure of ambient air and the changes in the humidity of air, a careful study has been made by J. C. Owens, which is published in "Applied Optics", 1964, No. 1, and it is known that changes in the pressure or humidity of the ambient air causes changes in the index of refraction of the air. Since, in such case, the index of refraction of the glass material has not substantially been changed, the relative index of refraction at the refracting surface will be changed.
The relative index of refraction n defined by the glass material and the air can be given by: EQU n=n.sub.G /n.sub.A
where n.sub.G is the absolute index of refraction of the glass material and n.sub.A is the absolute index of refraction of the air. The index n.sub.A .apprxeq.1, so that the amount of change .DELTA.n in the relative index of refraction n when the absolute index of refraction n.sub.A is changed by an amount .DELTA.n.sub.A can be given by: EQU .DELTA.n.apprxeq.n.sub.G .multidot..DELTA.n.sub.A
Usually, n.sub.G is approximately 1.5. From this, it follows: EQU .DELTA.n=1.5 .DELTA.n.sub.A
From the above, it is found that the change in the index of refraction of the air causes the change in the relative index of refraction of the glass material and air by an amount which is one and half times greater than the amount of change in the index of refraction of the air itself. For example, when the ambient air pressure changes through 5 mmHg, the index of refraction of the air changes approx. by 1.8.times.10.sup.-6. This corresponds to 2.7.times.10.sup.-6 of the relative index of refraction of the glass and air and corresponds to the focus displacement of approx. 0.5-1.5 microns (the displacement being different depending on the characteristics of the projection optical systems). Such amount of focus displacement could not be ignored in respect to the performance of the exposure apparatus, as will be apparent from the fact that the depth of focus is within a range of .+-.0.9 micron as described in the foregoing.
The inventors have found that, when such change actually occurs, there also occurs a magnification error in the pattern projected on the wafer surface by the projection optical system. Usually, the semiconductor device is formed by superposing different patterns on the wafer. If the magnification of pattern projection varies for different circuit patterns, it would be difficult to accurately overlay these circuit patterns on the wafer. This results in disadvantageous degradation of the reliability of the semiconductor device.
In the lens projection type exposure apparatuses, as described above, changes in the external environments such as ambient pressure, temperature, humidity, etc. cause inconveniences due to the focus error and magnification error. Traditionally, these errors are corrected by executing an exposure examination once per three days and by performing a complicated adjustment of the exposure apparatus to ensure optimum pattern writing on the wafer.
The same assignee as of the subject application filed on Dec. 8, 1983 a Japanese Patent Application No. 58-230578 in which the assignee has proposed to detect the temperature of a projection optical system and to change the distance between the projection optical system and a wafer surface in accordance with the change of the focus position of the projection optical system.